[RFC PATCH 1/2] mm,drm/ttm: Block fast GUP to TTM huge pages

[Thomas Hellström (Intel)]

On 3/23/21 8:52 PM, Williams, Dan J wrote:
> On Sun, 2021-03-21 at 19:45 +0100, Thomas Hellström (Intel) wrote:
>> TTM sets up huge page-table-entries both to system- and device
>> memory,
>> and we don't want gup to assume there are always valid backing struct
>> pages for these. For PTEs this is handled by setting the pte_special
>> bit,
>> but for the huge PUDs and PMDs, we have neither pmd_special nor
>> pud_special. Normally, huge TTM entries are identified by looking at
>> vma_is_special_huge(), but fast gup can't do that, so as an
>> alternative
>> define _devmap entries for which there are no backing dev_pagemap as
>> special, update documentation and make huge TTM entries _devmap,
>> after
>> verifying that there is no backing dev_pagemap.
> Please do not abuse p{m,u}d_devmap like this. I'm in the process of
> removing get_devpagemap() from the gup-fast path [1]. Instead there
> should be space for p{m,u}d_special in the page table entries (at least
> for x86-64). So the fix is to remove that old assumption that huge
> pages can never be special.
>
> [1]:
> http://lore.kernel.org/r/161604050866.1463742.7759521510383551055.stgit@dwillia2-desk3.amr.corp.intel.com
>
Hmm, yes with that patch it will obviously not work as intended.

Given that, I think we'll need to disable the TTM huge pages for now 
until we can sort out and agree on using a page table entry bit.

Thanks,

/Thomas